This invention relates to continuous wave (CW) radar systems and more particularly to means for cancelling AM noise from such systems.
In the past, CW radars have generally employed spaced-apart transmitting and receiving antennas in order to eliminate problems associated with AM noise. Specifically, radomes and other in-close reflecting objects return fairly high level AM noise contaminated signals to the antenna where a single antenna is used. It has been suggested that the use of an active RF cancellation scheme in which the magnitude and phase of reflections from close-in objects such as a radome be measured in the line leading to the receiver and a signal of equal amplitude and opposite phase be automatically generated and inserted to cancel the undesired reflection. The bandwidth of the RF cancellation path is broad enough so that AM source noise riding on the carrier would also be cancelled, and hence not able to blind the receiver. Although this scheme has worked satisfactorily under laboratory conditions, a fixed loss of three to four dB must be accepted in economically practical versions of the system to provide and process the signal which is to be reinserted for cancellation. In addition, the various microwave integrated circuit devices used in employing this scheme have exhibited significant insertion loss, which when added to the fixed processing loss mentioned above might total six dB. Unfortunately, the power levels of presently available solid state devices are not high enough to provide significant radar range in the face of such losses.
In other CW radar systems a balanced mixer is employed in the main receiver to eliminate AM noise present on the local oscillator (LO) signal. Because of the presence of reflection from the duplexer-radome combination in the single aperture FM-CW radar, however, source noise of sufficient level to jam the receiver will enter the RF port of the mixer directly along with the desired target returns. This particular condition cannot be remedied by the use of a balanced mixer.